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