Perpetual motion refers to the movement of objects that keeps going forever in a system that isn't disturbed. A perpetual motion machine is an imaginary machine that can work forever without needing energy from outside. This kind of machine cannot exist because it would break the first and/or second laws of thermodynamics. These laws work the same way no matter how big or small the system is. Machines that take energy from limited sources cannot keep working forever because they rely on energy stored in the source, which will eventually run out. A common example is devices that use energy from ocean currents, which comes from the Sun. The Sun itself will one day burn out.

History

The history of perpetual motion machines began during the Middle Ages. For many years, it was uncertain if these machines could work. Later, modern thermodynamics theories proved that perpetual motion devices are impossible. Even so, people have continued trying to build them throughout history. Today, some inventors use different terms, like "over unity," to describe their designs.

Basic principles

Leonardo da Vinci once wrote, "Oh ye seekers after perpetual motion, how many vain chimeras have you pursued? Go and take your place with the alchemists." This quote reflects the long history of failed attempts to create machines that operate without an external energy source.

Scientists agree that perpetual motion in an isolated system violates the first or second law of thermodynamics, or both. The first law states that energy cannot be created or destroyed, only changed from one form to another. The second law explains that heat naturally flows from hotter to colder areas, and that all processes involve some energy loss, such as friction. For example, no heat engine can be more efficient than a Carnot engine operating between the same two temperatures.

Some machines, like clocks powered by differences in air pressure or temperature, appear to work without an obvious energy source. However, they still rely on external energy, even if it is not immediately visible. These devices do not truly break the laws of thermodynamics.

Machines that use energy from long-term sources, such as ocean currents, eventually stop when their energy supply runs out. They are not perpetual motion machines because they depend on outside energy and are not isolated systems.

Perpetual motion machines are often classified based on which thermodynamic law they claim to break. "Epistemic impossibility" refers to events that cannot occur according to current scientific understanding. The conservation of energy, supported by Noether's theorem, is deeply tied to the unchanging nature of physical laws over time. If energy conservation were violated, it would require a fundamental change in physics, which has not been observed.

Scientists study distant stars and other phenomena to confirm that physical laws have remained consistent for billions of years. The principles of thermodynamics are so well-supported by evidence that proposals for perpetual motion machines are dismissed by experts. These designs, however, often help scientists better understand physical laws. For instance, the Brownian ratchet thought experiment, first proposed in 1900, was not fully explained until 1912.

Sir Arthur Eddington once said that the second law of thermodynamics holds the highest authority among natural laws. If a theory contradicts this law, it is considered impossible to fix.

In the 1800s, Henry Dircks criticized those who continued to pursue perpetual motion, calling their efforts foolish and misguided. He wrote, "There is something lamentable… in pursuing the visionary schemes of past ages with dogged determination."

Many perpetual motion ideas, such as those involving magnets or gravity, have appeared repeatedly. For example, John Wilkins proposed using magnets to pull a ball up a ramp, but the design fails because the magnet's strength must be balanced to allow the ball to return. Similarly, Bhaskara's wheel, which relies on weights shifting to create motion, does not work because energy is always lost during the process.

Gravity, like magnetism, acts at a distance but requires energy input to produce motion. For example, lifting an object to gain potential energy requires effort, and some energy is always lost in the process. Machines that depend on gravity, such as Bhaskara's wheel, cannot operate indefinitely because they cannot avoid energy loss.

Patents

Proposals for machines that cannot function, such as perpetual motion machines, are so common that the United States Patent and Trademark Office (USPTO) has created a rule: it will not grant patents for perpetual motion machines without a working model. The USPTO Manual of Patent Examining Practice explains:

Usually, a model is not needed to show that a device works, except in cases involving perpetual motion. If the operability of a device is questioned, the applicant must prove it works to the examiner’s satisfaction, using any method they choose.

A patent application may be rejected because the invention lacks usefulness, including cases where the device is claimed to be inoperable, such as perpetual motion machines. A rejection under 35 U.S.C. 101 for lack of usefulness should not be based on the invention being silly, dishonest, or against public policy.

Filing a patent application is a routine task. The USPTO will not refuse applications for perpetual motion machines. However, the application will be accepted and likely rejected later by a patent examiner after a formal review. Even if a patent is granted, it does not mean the invention works. It only means the examiner believes it works or could not determine why it might not.

The United Kingdom Patent Office has specific rules about perpetual motion. Section 4.05 of the UKPO Manual of Patent Practice states:

Processes or items that are said to work in a way that clearly goes against well-known physical laws, such as perpetual motion machines, are considered to lack industrial usefulness.

Examples of the UK Patent Office rejecting patent applications for perpetual motion machines include:

The European Patent Classification (ECLA) includes categories for patent applications related to perpetual motion systems: ECLA classes "F03B17/04: Alleged perpetua mobilia" and "F03B17/00B: [… machines or engines] (with closed loop circulation or similar : … Installations wherein the liquid circulates in a closed loop; Alleged perpetua mobilia of this or similar kind".

Apparent perpetual motion machines

A perpetual motion machine can only be described in a system that has set limits and does not interact with anything else. However, true isolated systems do not exist because of tiny, unpredictable movements in the quantum world. Because of this, "perpetual motion" in this article is better described as a "perpetual motion machine," since a machine is a device that performs tasks using mechanical, electrical, or electronic methods, while "motion" simply means movement, like the random movement of particles in Brownian motion.

Even though some ideas and designs suggest "perpetual motion" on a large scale, closer examination shows that these devices actually use natural resources or hidden energy, such as changes in the state of water or other liquids, or small temperature differences. These devices cannot operate forever, and it is not possible to extract useful work from them.

Examples of such devices include:

In 2016, scientists discovered new forms of matter called time crystals. These consist of atoms that move in repeating patterns on a microscopic level, which fits the literal meaning of "perpetual motion." However, these are not considered perpetual motion machines in the traditional sense, nor do they break the laws of thermodynamics. This is because time crystals exist in their lowest energy state, meaning no energy can be taken from them—they move without requiring energy.

Sometimes, imaginary experiments seem to suggest that perpetual motion might be possible using known physical processes. However, in every case, a mistake or flaw is found when all the relevant physics is fully considered. Examples of these include:

Conspiracy theories

Perpetual motion machines are not considered real science by many experts. However, some people believe that these machines are being kept secret by companies or governments. They think this is because such machines could provide energy at a low cost, which would take away control over money and power from those groups.