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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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second. At the same time, the arc of the human
lifespan is getting longer: 67 years is the
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global average, up from just 20 years in the
Stone Age.
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Modern science provides a humbling perspective.
Our lives, indeed even that of the human species,
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are just a blip compared to the Earth, at
4.5 billion years and counting, and the universe,
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at 13.7 billion years.
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It now appears the entire cosmos is living
on borrowed time. It may be a blip within
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a much grander sweep of time. When, we now
ask, will time end?
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Our lives are governed by cycles of waking
and sleeping, the seasons, birth and death.
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Understanding time in cyclical terms connects
us to the natural world, but it does not answer
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the questions of science.
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What explains Earth’s past, its geological
eras and its ancient creatures? And where
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did our world come from? How and when will
it end? In the revolutions spawned by Copernicus
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and Darwin, we began to see time as an arrow,
in a universe that’s always changing.
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The 19th century physicist, Ludwig Boltzmann,
found a law he believed governed the flight
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of Time’s arrow. Entropy, based on the 2nd
law of thermodynamics, holds that states of
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disorder tend to increase.
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From neat, orderly starting points, the elements,
living things, the earth, the sun, the galaxy.
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are all headed eventually to states of high
entropy or disorder. Nature fights this inevitable
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disintegration by constantly reassembling
matter and energy into lower states of entropy
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in cycles of death and rebirth.
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Will entropy someday win the battle and put
the breaks on time’s arrow? Or will time,
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stubbornly, keep moving forward?
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We are observers, and pawns, in this cosmic
conflict. We seek mastery of time’s workings,
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even as the clock ticks down to our own certain
end. Our windows into the nature of time are
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the mechanisms we use to chart and measure
a changing universe, from the mechanical clocks
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of old, to the decay of radioactive elements,
or telescopes that measure the speed of distant
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Our lives move in sync with the 24-hour day,
the time it takes the Earth to rotate once.
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Well, it’s actually 23 hours, 56 minutes
and 4.1 seconds if you’re judging by the
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stars, not the sun. Earth got its spin at
the time of its birth, from the bombardment
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of rocks and dust that formed it. But it’s
gradually losing it to drag from the moon’s
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That’s why, in the time of the dinosaurs,
a year was 370 days, and why we have to add
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a leap second to our clocks about every 18
months. In a few hundred million years, we’ll
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gain a whole hour.
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The day-night cycle is so reliable that it
has come to regulate our internal chemistry.
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The fading rays of the sun, picked up by our
retinas, set our so-called “circadian rhythms”
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in motion. That’s when our brains begin
to secrete melatonin, a hormone that tells
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our bodies to get ready for sleep.
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Finally, in the light of morning, the flow
of melatonin stops. Our blood pressure spikes…
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body temperature and heart rate rise as we
move out into the world. Our days, and our
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lives, are short in cosmic terms. But with
our minds, we have learned to follow time’s
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trail out to longer and longer intervals.
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We know from precise measurements that the
Earth goes around the sun every 365.256366
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days. Much of the solar energy that hits our
planet is reflected back to space or absorbed
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by dust and clouds. The rest sets our planet
in motion.
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You can see it in the ebb and flow of heat
in the tropical oceans, the annual melting
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and refreezing of ice at the poles, or seasonal
cycles of chlorophyll production in plants
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on land and at sea. These cycles are embedded
in still longer Earth cycles. Ocean currents,
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