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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 destroyed.

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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.