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The universe as a whole evolves towards increasing entropy, or disorder -- a tendency physicists
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call the Second Law of Thermodynamics.
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This movement toward disorganization might lead you to think that organized structures
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– like, say, living beings – would never spontaneously come into existence.
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Of course entropy can go down in part of the universe -- you can trade a decrease in entropy
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in one place (like cooling water so it crystallizes into ice) for an equal or larger increase
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in entropy somewhere else (like heating the back of your fridge).
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Order increases here, but only at the cost of decreasing order there.
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But we can still ask: why do intricate, complex structures come into being in the universe,
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if the overall tendency is toward increasing disorder?
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The secret is that order and complexity are very different ideas.
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Entropy measures how many different ways you can make an arrangement of small-scale particles
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that have the same large-scale properties: like, 37 degrees celsius, brown hair, good
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at soccer, and so on.
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[There are lots of different ways!].
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Complexity, on the other hand, is a measure of how hard it is to describe a set of large-scale
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properties.
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Simple systems are easy to describe; complex systems require a lot more information.
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For example, take a cup filled with half coffee and half milk.
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It starts off in a state with relatively low entropy – you could swap coffee molecules
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with each other, or milk molecules with each other, without changing things substantially.
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But if you swapped coffee molecules with milk molecules that would be a noticeable change.
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It’s also a simple setup -- milk on top, coffee on the bottom.
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Now, as the milk and coffee begin to mix, entropy goes up – where they are mixed together,
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swapping some coffee molecules for milk molecules no longer makes much of a difference.
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But the system also becomes more complex - to describe what you see, you would have to specify
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exactly how all of those tendrils of milk and coffee intricately swirl into each other.
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Continuing on, entropy keeps going up, until the milk and coffee are completely mixed together
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and swapping any molecules of coffee and milk with any others doesn’t really make any
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difference at all.
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That’s equilibrium, where there are a huge number of arrangements of the molecules that
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look essentially the same.
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But this highly-mixed equilibrium is once again simple: it’s just a homogenous mixture
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of coffee and milk; no more complicated fractal swirly stuff.
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This general principle is borne out time and time again: while entropy increases, complexity
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initially grows, then decays.
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Complexity can be a natural step along the path to increasing entropy.
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The best example is the universe itself.
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The early universe was very smooth and very dense: that’s low-entropy, and also extremely simple.
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The far future will be smooth again, but very dilute: that’s high-entropy, and again extremely simple.
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It’s now, in the medium-entropy middle, that things look complex.
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Stars and galaxies and veins of minerals in rock and swirling clouds and amino acids and
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proteins and human beings and cats – we’re at the exciting, beautiful stage of the coffee
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mixing!
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But just as with the coffee and milk, in the far distant future complexity will decrease
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again, and complicated stuff like us will at last be simplified out of existence.
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Hey, Henry here, thanks for watching.
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This is the third video in a series about time and entropy made in collaboration with
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physicist Sean Carroll.
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The series is supported with funding from Google’s Making and Science initiative,
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which seeks to encourage more young people (and people of all ages) to learn about and
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fall in love with science and the world around them, and the videos are based off of Sean’s
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book “The Big Picture: On the Origins of Life, Meaning, and the Universe Itself,”
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which you can find online or in bookstores around the world.
