Around 1159 A.D.,
一位被称为 渊博的婆什迦罗 的数学家
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 bottomof each reservoir,
leaving one side of the wheelperpetually heavier than the other.
The imbalance would keep the wheel turning forever.
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 transformour relationship with energy.
For example, if you could builda perpetual motion machine
that included humans as part of itsperfectly 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 morefundamental laws of thermodynamics,
the branch of physics that describesthe 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 energythan you put in.
That rules out a usefulperpetual 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 variationson Bhaskara’s over-balanced wheel
with rolling ballsor weights on swinging arms.
None of them work.
The moving parts that make oneside of the wheel heavier
also shift its center of mass downwardbelow the axle.
With a low center of mass,
the wheel just swings back and forth like a pendulum,
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 strongenough 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 amountsof 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 efficientenergy 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’tunderstand about the universe.
Perhaps we’ll findnew exotic forms of matter
that’ll force us to revisit the lawsof thermodynamics.
Or maybe there’s perpetual motionon 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.