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Around 1159 A.D.,
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a mathematician called Bhaskara the Learned
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sketched a design for a wheel containing curved reservoirs of mercury.
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He reasoned that as the wheels spun,
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the mercury would flow to the bottom of each reservoir,
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leaving one side of the wheel perpetually heavier than the other.
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The imbalance would keep the wheel turning forever.
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Bhaskara's drawing was one of the earliest designs
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for a perpetual motion machine,
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a device that can do work indefinitely without any external energy source.
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Imagine a windmill that produced the breeze it needed to keep rotating.
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Or a lightbulb whose glow provided its own electricity.
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These devices have captured many inventors' imaginations
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because they could transform our relationship with energy.
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For example, if you could build a perpetual motion machine
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that included humans as part of its perfectly efficient system,
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it could sustain life indefinitely.
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There's just one problem.
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They don't work.
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Ideas for perpetual motion machines
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all violate one or more fundamental laws of thermodynamics,
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the branch of physics that describes the relationship
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between different forms of energy.
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The first law of thermodynamics says that energy can't be created or detroyed.
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You can't get out more energy than you put in.
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That rules out a useful perpetual motion machine right away
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because a machine could only ever produce as much energy as it consumed.
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There wouldn't be any left over to power a car or charge a phone.
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But what if you just wanted the machine to keep itself moving?
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Inventors have proposed plenty of ideas.
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Several of these have been variations on Bhaskara's over-balanced wheel
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with rolling balls or weights on swinging arms.
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None of them work.
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The moving parts that make one side of the wheel heavier
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also shift its center of mass downward below the axle.
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With a low center of mass,
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the wheel just swings back and forth like a pendulum,
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then stops.
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What about a different approach?
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In the 17th century, Robert Boyle came up with an idea
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for a self-watering pot.
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He theorized that capillary action,
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the attraction between liquids and surfaces
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that pulls water through thin tubes,
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might keep the water cycling around the bowl.
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But if the capillary action is strong enough to overcome gravity
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and draw the water up,
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it would also prevent it from falling back into the bowl.
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Then there are versions with magnets, like this set of ramps.
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The ball is supposed to be pulled upwards by the magnet at the top,
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fall back down through the hole,
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and repeat the cycle.
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This one fails because like the self-watering pot,
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the magnet would simply hold the ball at the top.
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Even if it somehow did keep moving,
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the magnet's strength would degrade over time
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and eventually stop working.
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For each of these machines to keep moving,
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they'd have to create some extra energy
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to nudge the system past its stopping point,
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breaking the first law of thermodynamics.
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There are ones that seem to keep going,
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but in reality, they invariably turn out to be drawing energy
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from some external source.
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Even if engineers could somehow design a machine
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that didn't violate the first law of thermodynamics,
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it still wouldn't work in the real world because of the second law.
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The second law of thermodynamics
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tells us that energy tends to spread out through processes like friction.
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Any real machine would have moving parts
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or interactions with air or liquid molecules
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that would generate tiny amounts of friction and heat,
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even in a vacuum.
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That heat is energy escaping,
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and it would keep leeching out,
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reducing the energy available to move the system itself
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until the machine inevitably stopped.
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So far, these two laws of thermodynamics
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have stymied every idea for perpetual motion
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and the dreams of perfectly efficient energy generation they imply.
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Yet it's hard to conclusively say we'll never discover a perpetual motion machine
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because there's still so much we don't understand about the universe.
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Perhaps we'll find new exotic forms of matter
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that'll force us to revisit the laws of thermodynamics.
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Or maybe there's perpetual motion on tiny quantum scales.
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What we can be reasonably sure about is that we'll never stop looking.
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For now, the one thing that seems truly perpetual is our search.
