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 leftover 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. None of them work.

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 of thermodynamics. 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.

Various kinds of Perpetual Motion Machines can be classified in the following categories and here we see, why they are doomed to fail given our current understanding of Science.

• A perpetual motion machine of the first kind produces work without the input of energy. It thus violates the first law of thermodynamics: the law of conservation of energy.
• A perpetual motion machine of the second kind is a machine which spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However, it does violate the more subtle second law of thermodynamics (see also entropy). The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, according to the second law of thermodynamics.
• A perpetual motion machine of the third kind is usually (but not always) defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). Such a machine should satisfy the following 3 properties, at the least.
  The machine should not have any “rubbing” parts: Any moving part must not touch other parts. This is because of friction that would be created between the two. This friction will ultimately cause the machine to lose its energy to heat.
  The machine must be operated inside a vacuum (no air): The reason for this has to do with the reason listed in number one. Operating the machine anywhere will cause the machine to lose energy due to the friction between the moving parts and air. Although the energy lost due to air friction is very small, remember, we are talking about perpetual motion machines here, if there is a loss mechanism, eventually, the machine will still lose its energy and run down (even if it takes a long, long time).
  The machine should not produce any sound: a Sound is also a form of energy; if the machine is making any sound, that means that it is also losing energy.
  It is impossible to make such a machine, as dissipation can never be completely eliminated in a mechanical system, no matter how close a system gets to this ideal.

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 a 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.

Following are some popular proposals for Perpetual Motion Machines, which may seem convincing in the first sight, but on detailed analysis, they contradict at least one of the Laws of Thermodynamics.

There are concepts and technical drafts that propose “perpetual motion”, but on closer analysis, it is revealed that they actually “consume” some sort of natural resource or latent energy, such as the phase changes of water or other fluids or small natural temperature gradients, or simply cannot sustain the indefinite operation. In general, extracting work from these devices is impossible.

Resource consuming

Some examples of such devices include:

• The drinking bird toy functions using small ambient temperature gradients and evaporation. It runs until all water is evaporated.
• A capillary action-based water pump functions using small ambient temperature gradients and vapor pressure differences. With the “Capillary Bowl”, it was thought that the capillary action would keep the water flowing in the tube, but since the cohesion force that draws the liquid up the tube in the first place holds the droplet from releasing into the bowl, the flow is not perpetual.

• A Crookes radiometer consists of a partial vacuum glass container with a lightweight propeller moved by (light-induced) temperature gradients.
• Any device picking up minimal amounts of energy from the natural electromagnetic radiation around it, such as a solar-powered motor.
• Any device powered by changes in air pressure, such as some clocks (Cox’s timepiece, Beverly Clock). The motion leeches energy from moving air which in turn gained its energy from being acted on.
• The Atmos clock uses changes in the vapor pressure of ethyl chloride with temperature to wind the clock spring.
• A device powered by radioactive decay from an isotope with a relatively long half-life; such a device could plausibly operate for hundreds or thousands of years.
• The Oxford Electric Bell and Karpen Pile, driven by dry pile batteries.

Low friction

• In flywheel energy storage, “modern flywheels can have a zero-load rundown time measurable in years”.
• Once spun up, objects in the vacuum of space — stars, black holes, planets, moons, spin-stabilized satellites, etc. — dissipate energy very slowly, allowing them to spin for long periods. Tides on Earth are dissipating the gravitational energy of the Moon/Earth system at an average rate of about 3.75 terawatts.
• In certain quantum-mechanical systems (such as superfluidity and superconductivity), very low friction movement is possible. However, the motion stops when the system reaches an equilibrium state (e.g. all the liquid helium arrives at the same level.) Similarly, seemingly entropy-reversing effects like superfluids climbing the walls of containers operate by ordinary capillary action.

Thought experiments

In some cases, a thought experiment appears to suggest that perpetual motion may be possible through accepted and understood physical processes. However, in all cases, a flaw has been found when all of the relevant physics is considered. Examples include:

• Maxwell’s demon: This was originally proposed to show that the Second Law of Thermodynamics applied in the statistical sense only, by postulating a “demon” that could select energetic molecules and extract their energy. Subsequent analysis (and experiment) have shown there is no way to physically implement such a system that does not result in an overall increase in entropy.
• Brownian ratchet: In this thought experiment, one imagines a paddle wheel connected to a ratchet. Brownian motion would cause surrounding gas molecules to strike the paddles, but the ratchet would only allow it to turn in one direction. A more thorough analysis showed that when a physical ratchet was considered at this molecular scale, Brownian motion would also affect the ratchet and cause it to randomly fail to result in no net gain. Thus, the device would not violate the Laws of thermodynamics.
• Vacuum energy and zero-point energy: In order to explain effects such as virtual particles and the Casimir effect, many formulations of quantum physics include background energy which pervades empty space, known as vacuum or zero-point energy. The ability to harness zero-point energy for useful work is considered pseudoscience by the scientific community at large. Inventors have proposed various methods for extracting useful work from zero-point energy, but none have been found to be viable, no claims for extraction of zero-point energy have ever been validated by the scientific community, and there is no evidence that zero-point energy can be used in violation of conservation of energy.

References
1. Why don’t perpetual motion machines ever work? — Netta Schramm
2. Science Explained: The Physics of Perpetual Motion Machines
3. Perpetual Motion.