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The earliest time measurements were
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observations of cycles of the natural world,
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using patterns of changes from day to night
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and season to season to build calendars.
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More precise time-keeping, like sundials
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and mechanical clocks, eventually came along
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to put time in more convenient boxes.
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But what exactly is it that we’re measuring?
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Is time something that physically exists,
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or is it just in our heads?
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At first the answer seems obvious—
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of course time exists;
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it constantly unfolds all around us,
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and it’s hard to imagine the universe without it.
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But our understanding of time started
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getting complicated thanks to Einstein.
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His theory of relativity tells us that time
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passes for everyone, but doesn’t always pass
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at the same rate for people in different situations,
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like those travelling close to the speed of light
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or orbiting a supermassive black hole.
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Einstein resolved the malleability of time
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by combining it with space to define space-time,
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which can bend, but behaves
in consistent, predictable ways.
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Einstein’s theory seemed to confirm that time
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is woven into the very fabric of the universe.
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But there’s a big question it didn’t fully resolve:
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why is it we can move through space in any direction,
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but through time in only one?
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No matter what we do, the past is always,
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stubbornly, behind us.
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This is called the arrow of time.
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When a drop of food coloring is
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dropped into a glass of water,
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we instinctively know that the coloring
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will drift out from the drop,
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eventually filling the glass.
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Imagine watching the opposite happen.
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Here, we’d recognize time as unfolding backwards.
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We live in a universe where the food coloring
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spreads out in the water,
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not a universe where it collects together.
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In physics, this is described by
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the Second Law of Thermodynamics,
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which says that systems will gain disorder,
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or entropy, over time.
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Systems in our universe move from order to disorder,
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and it is that property of the universe
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that defines the direction of time’s arrow.
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So if time is such a fundamental property,
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it should be in our most fundamental equations
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describing the universe, right?
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We currently have two sets of
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equations that govern physics.
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General relativity describes the
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behavior of very large things,
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while quantum physics explains the very small.
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One of the biggest goals in theoretical physics
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over the last half century has been reconciling
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the two into one fundamental “theory of everything."
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There have been many attempts
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—none yet proven—
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and they treat time in different ways.
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Oddly enough, one contender called the Wheeler-DeWitt
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equation, doesn’t include time at all.
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Like all current theories of everything,
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that equation is speculative.
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But as a thought experiment,
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if it or a similarly time-starved equation
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turned out to be true, would that mean
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that time doesn’t exist, at the most fundamental level?
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Could time just be some sort of illusion generated
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by the limitations of the way
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we perceive the universe?
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We don’t yet know, but maybe that’s
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the wrong way of thinking about it.
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Instead of asking if time exists as a fundamental property,
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maybe it could exist as an emergent one.
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Emergent properties are things that don’t exist
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in individual pieces of a system,
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but do exist for the system as a whole.
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Each individual water molecule doesn’t have a tide,
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but the whole ocean does.
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A movie creates change through time by using
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a series of still images that appear to have a fluid,
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continuous change between them.
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Flipping through the images fast enough,
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our brains perceive the passage of time
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from the sequence of still images.
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No individual frame of the movie changes
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or contains the passage of time,
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but it’s a property that comes out of how
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the pieces are strung together.
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The movement is real, yet also an illusion.
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Could the physics of time somehow be a similar illusion?
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Physicists are still exploring these and other questions,
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so we’re far from a complete explanation.
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At least for the moment.7302
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